In forming fine patterns on electronic devices such as semiconductor integrated circuits and liquid crystal displays, a method has been used which transfers a pattern, having a size about 4 to 5 times the size of an actual pattern on the wafer, on a photo-mask or a reticle (both to be referred to as a “reticle” hereinafter) onto a substrate subject to exposure such as a wafer, reducing the pattern by using a reduction-projection exposure apparatus such as a stepper.
In projection exposure apparatuses for transferring a pattern, exposure apparatuses of a newer generation use exposure light having a wavelength shorter than that of the previous generation, corresponding to semiconductor integrated circuits becoming finer. Although, at present, KrF excimer laser having a wavelength of 248 nm is mainly used, ArF excimer laser having a shorter wavelength of 193 nm is going to be used. And projection exposure apparatuses that utilize F2 laser having a wavelength of 157 nm or Ar2 laser having a wavelength of 126 nm have been suggested.
Such light in the wavelength band of 120 to 200 nm is vacuum ultraviolet light and has a poor transmittance to optical glass. Usable materials of lenses and reticles for a VUV exposure apparatus, which utilizes vacuum ultraviolet light (VUV) as exposure illumination light, are limited to crystal such as fluorite, magnesium fluoride and lithium fluoride. Moreover, because the energy absorption of exposure light by gas such as oxygen, steam and hydro carbon (referred to as absorbent gas) is extremely large, gas in the optical path of exposure light needs to be replaced with low-absorbent gas, which absorbs little of the exposure light energy, so as to remove absorbent gas from the optical path.
Because the energy absorption of vacuum ultraviolet light by oxygen is extremely large, the average concentration of oxygen in the optical path needs to be restricted to a value smaller than about 1 ppm. Specially, because the total length of the optical path of a so-called illumination optical system from the light source to the reticle is long, the oxygen concentration needs to be restricted to an even smaller value. Meanwhile, because the length of the optical path from the projection optical system to the wafer is short, e.g. several to several tens mm, a small quantity of absorbent gas in this path has no significant impact on the absorption.
However, because an exposure apparatus used in mass-production of integrated circuits such as LSI's needs to expose about 80 wafers per hour to exposure light, there is a large possibility of absorbent gas from the outside getting into the optical path of exposure light from the projection optical system to the wafer upon the frequent replacement of wafers.
In addition, although, also in the optical path near the reticle, the oxygen concentration needs to be kept lower than several ppm, there is a possibility of absorbent gas from the outside getting into the optical path when replacing a reticle in the apparatus.
If absorbent gas gets into the optical paths of exposure light due to those factors, absorption rate and thus transmission fluctuate to some extent depending on the concentration of the absorbent gas, and exposure light energy on the wafer (a substrate subject to exposure) becomes unstable. Furthermore, if there is water or organic pollutants in the optical paths, a small quantity of such pollutants are very likely to stick to the surfaces of optical elements. Because the pollutants greatly absorb vacuum ultraviolet light (exposure light), transmission in the optical systems decreases due to the small quantity of pollutants sticking to the surfaces of the optical elements. Meanwhile, by irradiating the surfaces of the optical elements with vacuum ultraviolet light, organic substances are cut into smaller pieces and removed from the surfaces by the energy of the ultraviolet light, and the transmission of the exposure light increases. Therefore, pollutants in the optical paths cause variation of exposure light transmission.
This invention was made under such circumstances, and a first purpose of this invention is to provide an exposure apparatus and exposure method that can highly accurately transfer a pattern using exposure light in a vacuum ultraviolet band.
Moreover, a second purpose of this invention is to provide an exposure apparatus and exposure method that can suppress decrease and variation of exposure light transmission due to absorbent gas and pollutants in optical paths of exposure light.
A third purpose of this invention is to provide a device manufacturing method that can improve productivity of highly integrated devices.